Ahmadreza Amirahmadi

Design and Implementation of Three-Phase Two-Stage Grid-Connected Module Integrated Converter

Module integrated converters (MICs) in single phase have witnessed recent market success due to unique features such as improved energy harvest, improved system efficiency, lower installation costs, plug-and-play operation, and enhanced flexibility and modularity. The MIC sector has grown from a niche market to mainstream, especially in the United States. Assuming further expansion of the MIC market, this paper presents the microinverter concept incorporated in large size photovoltaic (PV) installations such as megawatts (MW)-class solar farms where a three-phase ac connection is employed. A high-efficiency three-phase MIC with two-stage zero voltage switching (ZVS) operation for the grid-tied PV system is proposed which will reduce cost per watt, improve reliability, and increase scalability of MW-class solar farms through the development of new solar farm system architectures. The first stage consists of a high-efficiency full-bridge LLC resonant dc-dc converter which interfaces to the PV panel and produces a dc-link voltage. A center points iteration algorithm developed specifically for LLC resonant topologies is used to track the maximum power point of the PV panel. The second stage is comprised of a three-phase dc-ac inverter circuit which employs a simple soft-switching scheme without adding auxiliary components. The modeling and control strategy of this three-phase dc-ac inverter is described. Because the dc-link capacitor plays such an important role for dual-stage MIC, the capacitance calculation is given under type D voltage dip conditions. A 400-W prototype was built and tested. The overall peak efficiency of the prototype was measured and found to be 96% with 98.2% in the first stage and 98.3% in the second stage.

A Center Point Iteration MPPT Method With Application on the Frequency-Modulated LLC Microinverter

Maximum power point tracking (MPPT) is an essential technique to harvest PV power under varying environments. Perturb and observe (P& O) algorithms are the most broadly used MPPT due to their effectiveness and simplicity. However, it is difficult to balance the tracking speed and oscillation requirements in the conventional P& O with fixed perturb. Adaptive P& O techniques have been proposed as a solution to these problems. However, they are based on duty cycle modulation for conventional pulse width modulation converters. None of them deal with the variable frequency modulation for resonant converters. In this paper, a center point iteration MPPT is proposed, with its variable perturb in frequency. The proposed scheme overcomes the drawbacks of conventional P& O with a simple calculation. Moreover, it is suitable for various power curves, especially the LLC microinverter power curves, which may confuse conventional MPPT algorithms. The effectiveness of the proposed MPPT method was verified in theory. A 300 W prototype was constructed, and the experimental results verified the effectiveness of the proposed center point iteration MPPT. An advanced version was also introduced in order to accelerate the tracking speed.

Hybrid ZVS BCM Current Controlled Three-Phase Microinverter

This paper presents a new zero-voltage switching (ZVS) control method that is suitable for low-power applications such as three-phase microinverters. The proposed hybrid control method increases the efficiency and power density of the microinverters and features both reduced number of components and easy digital implementation. ZVS is achieved by controlling the inductor current bidirectional in every switching cycle and results in lower switching losses, higher operating frequency, and reduced size and cost of passive components, especially magnetic cores. A 400 W prototype of a three-phase inverter and its hybrid control system have been designed and tested under different conditions to verify the effectiveness of the controller. Efficiency measurement and comparison of the three different current modulation schemes have been conducted, and the inverter exhibits peak efficiency of 98.4% with fixed reverse current boundary conduction mode modulation.

A Boundary-Mode Forward-Flyback Converter With an Efficient Active LC Snubber Circuit

This paper describes a boundary-mode forward-flyback converter (BMFFC) with zero-voltage switching that is able to process power efficiently. The theoretical analysis and operating principle of the BMFFC are presented in detail. A nondissipative LC snubber that recycles energy to the input source is employed in order to suppress the voltage spike caused by the leakage inductance of the transformer. The relatively large snubber capacitor also significantly reduces turn-off loss. Following a detailed design procedure, a 200 W prototype with a 25-50 V dc input and 230 V dc output was constructed and tested in order to evaluate the performance of the BMFFC.

High Efficiency Dual-Mode Current Modulation Method for Low-Power DC/AC Inverters

Boundary conduction mode (BCM) zero voltage switching (ZVS) current control is a promising soft switching method for microinverter applications. In this letter, different BCM ZVS current control modulation schemes are compared based on power losses breakdown, switching frequency range, and current quality. Compared to continuous conduction mode current control, BCM ZVS control decreases MOSFET switching losses and filter inductor conduction losses but increases MOSFET conduction losses and inductor core losses. Based on the loss analysis, a dual-mode current modulation method combining ZVS and zero current switching schemes is proposed to improve the efficiency of the microinverter. The experimental results show that by using this proposed current modulation scheme, higher efficiency of 0.5% can be achieved with no additional cost for a 400-W three-phase microinverter.

A comparison of soft and hard-switching losses in three phase micro-inverters

The efficiency of a PV grid-tie micro-inverter is of critical concern to its application. Three representative three-phase voltage source micro-inverter topologies have been selected to make loss comparisons: A Hard Switching Inverter (HSI), an Auxiliary Resonant Commutated Pole Inverter (ARCPI), and an Actively Clamped Resonant DC-link Inverter (ACRDCLI). Loss composition and estimation are given in detail for 350W micro-inverters. It is pointed out that the HSI has outstanding advantages in both efficiency and cost. The ARCPI losses are lower than the other two topologies. But the auxiliary circuits are costly and the controlling and driving circuits are complicated. The ACRDCLI has a simple structure but high DC-link losses. If an improved control scheme is used, the DC-link losses of the ACRDCLI can be reduced. This paper compares these advantages and limitations for assessing the selection of soft-switching topologies for micro-inverters.

Operation analysis and numerical approximation for the LLC DC-DC converter

The LLC resonant converter has the advantages of achieving high power efficiency for wide input voltage and load range, which makes it a popular candidate in high power density and high efficiency power converter applications. However, the complex interaction between resonant components makes it hard to analysis. The fundamental harmonic approximation (FHA) is widely adopted as a simplified analysis model. Yet, it has poor accuracy in gain prediction and cannot explain LLCs various operation modes. In this paper, a detailed mode analysis is developed that can offer highly accurate description on the resonant behavior, and the mode distribution under different load and frequency is illustrated and discussed. Moreover, a numerical approach is developed based on the mode model to provide precise peak gain points estimation. An experimental prototype is built to verify the analysis.

Variable boundary dual mode current modulation scheme for three-phase micro-inverter

A traditional half-bridge three-phase inverter topology can achieve zero voltage switching by implementing BCM operation which requires no additional power devices or magnetic components. In this paper a variable boundary dualmode current modulation method that combines ZVS and ZCS is proposed in order to improve the efficiency of the BCM current controlled micro-inverter. Decreasing output current moves the ZVS/ZCS boundary toward the peak of the line half cycle which reduces the switching losses at light loads. Increasing output current will move this boundary toward the line zero crossing which reduces conduction losses at heavier loads. The experimental results show that using the proposed current modulation scheme peak efficiency of 98.7% for a 400W three-phase micro-inverter can be achieved with no additional cost.

Zero voltage switching Forward-Flyback Converter with efficient active LC snubber circuit

This paper describes a Boundary Mode Forward-Flyback Converter (BMFFC) with zero-voltage switching that is able to process power efficiently. The theoretical analysis and operating principle of the BMFFC are presented in detail. A non-dissipative LC snubber that recycles energy to the input source is employed in order to suppress the voltage spike caused by the leakage inductance of the transformer. The relatively large snubber capacitor also significantly reduces turn-off loss. Following a detailed design procedure, a 200W prototype with a 25-50VDC input and 230VDC output was constructed and tested in order to evaluate the performance of the BMFFC.

Hybrid control of BCM soft-switching three phase micro-inverter

A new hybrid control of soft switching boundary conduction mode (BCM) inverter that features an easy digital implementation and reduced number of analog components is presented in this paper. A traditional half-bridge three-phase inverter topology can realize soft switching through implementing BCM operation, thus no additional power devices or magnetic components are required. The proposed hybrid method facilitates the control of BCM inverter by taking full advantage of the DSP resources. The simulation was carried out to verify the proposed control technique. A brief analysis of current modulation of soft switching BCM inverter is given in the paper. A 400W 120Vac three-phase prototype was built. The experimental results show that the soft switching can be realized under different load conditions and the peak efficiency can reach up to 96.5%.